1. Field of the Invention
The present invention relates to a method and apparatus for aligning image sensors with an optical system in an optical apparatus such as camera that employs image sensors for automatic focusing.
2. Description of the Related Art
The operating principle of automatic focusing in an optical apparatus such as camera is well known An optical system in the optical apparatus focuses an image of an object individually on linear image sensors. Each image sensor generates a respective set of visual data and these sets of visual data from the image sensors are compared with one another for detecting a value corresponding to the relative position or distance of the object with respect to the optical apparatus. Based on deviation of the detected value from a reference value, the distance of the object or the extent and direction of deviation of the object from a focal point of the optical system is determined.
The linear image sensors are typically constructed with charge-coupled devices (CCDs) or photodiode arrays. Usually a pair of linear image sensors are incorporated into a semiconductor integrated circuit chip. The physical arrangement of the pair of linear image sensors in the semiconductor chip largely depends on the arrangement of the optical system. The optical system focuses an image corresponding to an object on each of the pair of image sensors Typically, the pair of image sensors are disposed apart from each other longitudinally in a row in the semiconductor chip.
Regardless of whether the optical system is incorporated in or separated from the optical apparatus, the optical system usually includes a pair of imaging means or small lenses for receiving the image from the object. The optical system then focuses the image on each of the image sensors in a semiconductor device. The semiconductor device is positioned close to the focal point of the imaging means.
The optical system is usually assembled as an optical module into the optical apparatus, and high precision is required to mechanically couple the optical system or module to the pair of linear image sensors in the semiconductor device. There are several positional variables that have to be precisely controlled to attain such high precision mechanical coupling, in reference to the semiconductor chip: x and y-directions parallel to a planar surface of the semiconductor chip, a z-direction normal to the planar surface, and a direction of tilting angle .theta. around z-axis. In practice, the precision required in the z-direction does not pose a significant problem. However, the precision required in the x- and y-directions, poses the most significant problem, and in .theta. the next most significant problem.
To resolve these problems, an object or marker having a distinct pattern is placed at a specified distance from the optical system. The optical system receives an image of the object and produces the image on each of the pair of image sensors. A value corresponding to the distance of the object from the optical system or deviation from a focal point of the optical system is detected based on visual data obtained from each of the image sensors. In response to the detection, the semiconductor device is accordingly moved by small increments for alignment with the optical system until it reaches a point where the detected value coincides with a reference corresponding to the position of the object, and the semiconductor device is fixed securely at that point to the optical system.
The prior art method described above is probably one of the most reliable approaches conceivable since it provides a precision alignment on the basis of the result of the detection, which is an object of using the image sensors. This method, however, has a serious drawback in that the alignment precision obtained in this manner is not high enough for certain applications. In order to further increase alignment precision, the result of the detection or the detected value has to change significantly with a small change in the positional relation between the image sensors and the optical system. Conventionally, alignment precision can be obtained to the order of one unit, which is equivalent to a pitch between two adjacent photosensors in each image sensor.
Alignment precision on the order of one unit may be sufficient in many applications. However, in situation where the result of detection is obtained every fraction or less of an unit, alignment between the image sensors and the optical system must improve by at least an order of magnitude (i.e., to one tenth of the order of one unit). To satisfy this need, it has been attempted to substitute the visual data obtained from the image sensors for the result of the detection and to improve alignment precision by applying interpolation to the visual data. It has been proven, however, that this technique is still insufficient to improve precision to the order of one tenth of a unit. This would be partly because the visual data can only be obtained via photosensors and partly because the image produced on each image sensor may be slightly blurred.